Somehow in yesterday's entry on water purity I managed to completely forget to mention the flip side of the coin: The Washington, D.C. Water and Sewer Authority (DCWASA) recently determined that two-thirds of the DC-area residences they tested had tap water that exceeded the EPA lead limit. Apparently there's much disagreement over how big a problem this is; dust from lead paint is apparently a much greater hazard to children's health. But the guy I heard talking about this on NPR a couple weeks ago suggested that the sudden increase in lead levels could be due to a change in how the municipal water is processed—new chemicals leaching lead out of pipes—and that other cities are adopting that processing technology as well, possibly leading to increased lead levels in a lot of places.
There are apparently filters you can buy that will filter out lead, but I'm not sure whether all tap-water filters get the lead out (as it were).
I guess the moral is that water from any source can be dangerous. (And remember that dihydrogen monoxide is itself a potentially fatal chemical!) Filters good. Now that my tap water has stopped tasting terrible (so I've been drinking it again), I probably ought to get a filter of some sort.